Many manufacturing factory environments consist of spatially distributed processing tools, as opposed to sequential tools located along a linearly arranged assembly line. This is especially true for manufacturing environments where work in process, or a “work entity,” re-enters a tool after being processed by another tool or tools. Re-entry into the same tool avoids tool duplication, which is particularly important in environments where the capital cost of the tools is high.
A semiconductor manufacturing environment is an example of an environment where, due to high tool cost, a work entity enters a given tool, or type of tool, multiple times. Processing tools in a semiconductor manufacturing environment are typically spatially distributed in the factory according to function. Thus, the work flow resembles a chaotic movement of the work entity. With multiple work entities being operated upon and moving between multiple tools at the same time, the respective work flows intersect.
In modern factories, the progress of multiple work entities through the high number of manufacturing steps and associated tools is enabled by transport networks. Simultaneous processing of plural work entities, necessary to maximize usage of the factory tools and to maximize product output, results in highly complicated logistics. High efficiency and coordination in work entity movement is thus required. Without an efficient transportation network capable of rapid, real time response, bottlenecks in the work flow into or out of some process tools can develop (flow density), while other process tools are starved of work. Such an efficient transportation network thus must have high delivery capacity, high speed, and asynchronous capability by which work carriers can move independently of each other. The transport infrastructure is the enabling technology for such efficient logistics.
In a recursive process flow environment, such as within a semiconductor manufacturing environment, the simultaneous utilization of up to hundreds of individual process tools requires a logistics network that is capable of delivering the right work entity at the right time to each one of the tools. The higher the utilization of each processing tool, the higher the factory output, which simultaneously translates to the increased efficiency of business capital.
Conveyor systems are one particular type of transportation system used in contemporary factory environments. A conveyor network may be shared by several hundred moving work carriers concurrently dispatched to various tools. Delivery capacity will depend on flow density and conveyor speed. However, flow density and speed are limited by the additional requirements of zero tolerance for collisions between work entities within the conveyor system, and of a particulation free environment. Thus, a conflict arises between the above requirements.
A conveyor network typically has intersections, nodes, and branches to multiple locations in a factory. The open conveyor ends, at work processing locations, are the input and output ports for the conveyor transport domain. At these ports, work entities enter and leave the conveyor domain. When a work entity needs to travel from one of these ports to another in the prior art, a path needed to be cleared for the transit to satisfy the requirement of collision avoidance. Normally, external or centralized dispatch software arranges for such a transit by simultaneously controlling the movement of all other work entities that would otherwise interfere with the work entity in question. This dispatch software is complex, due to the aforementioned throughput requirements. The work entities need be moved concurrently with each other and at maximum rate without collisions.
In addition to the challenge of highly complex control in dense manufacturing environments, particulate generation by conveyor systems is of great concern in clean room environments. Thus, the efficiency of transport systems in such environments must be weighed against the opportunities for contamination.
Traditional roller conveyors have achieved extremely low particulate generation. However, such arrangements have not been able to achieve high acceleration of items or carriers transported thereon (generically referred to simply as “carriers” herein) from a stopped condition. This is not due to a lack of torque available for the drive rollers but instead due to the fact that when high starting torque is applied the roller wheels may slip and squeal. This is akin to auto tires squealing when accelerating too rapidly from a stop.
In certain embodiments, a hysteresis clutch has been utilized in conjunction with synchronous or stepper motor driven rollers or wheels, depending upon the embodiment, to eliminate such slippage between the carriers and drives. Hysteresis clutches enable asynchronous soft buffering, a process for moving carriers independent of each other and starting and stopping the carriers in a smooth fashion. However, while successful at preventing slippage, hysteresis clutches may make it difficult to achieve high rates of acceleration, including in the multiple g range. Very fast acceleration and deceleration are required in order to increase throughput and thus the density of carriers traveling on the soft buffered conveyor where carriers must never collide. Since the carriers move asynchronously, they need to stop fast and short of a collision with a downstream carrier to achieve increased density in a conveyor environment, as well as start fast so as to minimize interference with upstream carriers. Preferably, start and stop should occur within a line segment that is a little larger than the carrier.
Principles of physics dictate that the frictional force required to move an object on a surface is dependent on the normal force and the coefficient of friction for the materials. In other words, it is independent of the area in contact. However, with compressive materials, higher friction forces can be achieved by selectively increasing the surface contact. A result of this realization was the increased utilization of belts for carrier transport, instead of wheels with a rubber drive surface in contact with carriers. This increase in surface area contact in effect increased the friction force between driving and driven surfaces.
Unfortunately, simply disposing a driving belt on a respective set of wheels is not clean in terms of particulate generation, particularly with respect to that resulting from the use of driven and idler wheels alone. The particulation of the belts results primarily from interaction of the belt with the wheels below the belt, i.e. those supporting the weight of a carrier. Previous investigations into the source of particulate generation determined that in many cases the belt was not in continuous, full contact with the wheels below it due to machining tolerances in the wheels, the respective axles, and/or the rails that support the wheels. For example, some supporting idler wheels were found to be in constant contact with the overlying belt and thus were turning in concert with the belt while others started and stopped depending on when the belt touched them. The latter contact was haphazard, resulting in frictionally-induced spin up and stops of the supporting idler wheels. This effect was sometimes dependent upon whether a carrier was above the respective portion of belted conveyor.
In order to impart continuous contact between the belt and all of the wheels in a respective conveyor section, including the idler wheels, it was proposed that the belt be woven in a serpentine path between wheels, such as over two idler wheels and then down under the next. While successful in maintaining contact between the belt and all of the respective wheels, this resulted in an increased motor torque requirement, which also required increased electrical current and thus operational cost.
There remains the need for an optimized transport solution that results in high density, rapid, flexible, and asynchronous work entity transport, high delivery capacity, avoidance of work entity collisions, and low particulation, for use in clean room environments.